System, method and apparatus for improved electrical-to-optical transmitters disposed within printed circuit boards

ABSTRACT

The present invention provides a system, method and apparatus for improved electrical-to-optical transmitters ( 100 ) disposed within printed circuit boards ( 104 ). The heat sink ( 110, 200 ) is a thermal conductive material disposed within a cavity ( 102 ) of the printed circuit board ( 104 ) and is thermally coupled to a bottom surface ( 112 ) of the electrical-to-optical transmitter ( 100 ). A portion of the thermal conductive material extends approximately to an outer surface ( 120, 122  or  124 ) of a layer ( 114, 116  or  118 ) of the printed circuit board ( 104 ). The printed circuit board may comprise a planarized signal communications system or an optoelectronic signal communications system. In addition, the present invention provides a method for fabricating the heat sink wherein the electrical-to-optical transmitter disposed within a cavity of the printed circuit board is fabricated. New methods for flexible waveguides and micro-mirror couplers are also provided.

PRIORITY CLAIM AND U.S. GOVERNMENT GRANTS

This patent application is a U.S. non-provisional patent application ofU.S. provisional patent application Ser. No. 60/485,496 filed on Jul. 7,2003. This work was sponsored by United States Government grants fromDARPA.

FIELD OF THE INVENTION

The present invention relates generally to the field of optoelectronicsystems, and in particular, to a system, method and apparatus forimproved electrical-to-optical transmitters disposed within printedcircuit boards.

BACKGROUND OF THE INVENTION

Printed circuit boards (“PCBs”), multi-chip modules (“MCMs”), andsimilar structures having multiple integrated circuits mounted upontheir surfaces are used extensively in modem electronic devices andsystems. PCBs typically contain multiple conductive and dielectriclayers interposed upon each other, and interlayer conductive paths(referred to as vias), which may extend from an integrated circuitmounted on a surface of the PCB to one or more conductive layersembedded within the PCB. MCMs decrease the surface area by removingpackage walls between chips, improve signal integrity by shorteninginterconnection distances and remove impedance problems andcapacitances. MCMs and other similar structures typically have similarconfiguration and structure (e.g., a substrate comprising dielectric andconductive layers having interlayer vias). For ease of reference, allsuch structures shall hereafter be referred to as “boards”.

The speed and complexity of integrated circuits are increasing rapidlyas integrated circuit technology advances from very large scaleintegrated (“VLSI”) circuits to ultra large scale integrated (“ULSI”)circuits. As the number of components per chip, the number of chips perboard, the modulation speed and the degree of integration continue toincrease, electrical interconnects are facing fundamental limitations inareas such as speed, packaging, fan-out, and power dissipation. MCMtechnology has been employed to provide higher data transfer rates andcircuit densities. Conventional technologies based on electricalinterconnects, however, fail to provide requisite multi-Gbits/sec clockspeed in intra-MCM and inter-MCM applications.

Additionally, a printed circuit board may, in some instances, be quitelarge and the conductive paths therein can be several centimeters inlength. As conductive path lengths increase, impedances associated withthose paths also increase. This has a detrimental effect on the abilityof the system to transmit high speed signals. Although the use of copperand materials with lower dielectric constant materials can release thebottleneck in a chip level for the next several years, these materialswill not support interconnection speed over a few GHz even though chiplocal clock speeds are expected to constantly increase to 10 GHz by theyear 2011. It is therefore desirable that impedances of conductive pathsbe minimized; in order to, for example, transmit high speed signalsabove the 1 Gb/sec range.

High performance materials and advanced layout technologies, such asIMPS (Interconnected Mesh Power system), focus on signal integrity toprovide controlled impedance signal transmission with very low crosstalk. Such an electrical interconnection provides a 10 Gb/s link over adistance less than 20 m using coaxial cable. However, coaxial cabling isbulky; therefore, it is not suitable for high density interconnectionapplications. Electrical interconnects operating at high frequencyregion have many problems to be solved such as crosstalk, impedancematching, power dissipation, skew, and packing density. However, thereis a little hope to solve all of the problems. Optical interconnectiondoes, however, have several advantages, such as immunity to theelectromagnetic interference, independency to impedance mismatch, lesspower consumption, and high speed operation. Although the opticalinterconnects have great advantages compared to the copperinterconnection, they still have some difficulties regarding packaging,multi-layer technology, signal tapping, and re-workability.

The employment of optical interconnects will be one of the majoralternatives for upgrading the interconnection speed wheneverconventional electrical interconnection fails to provide the requiredbandwidth. In fact, several optical interconnect techniques, such asfree space, guided wave, board level, and fiber array interconnectionshave been introduced for system level applications. Although thesetechniques successfully demonstrated high speed optical interconnection,they continue to have packaging difficulties.

Machine to machine interconnection has already been significantlyreplaced by optical means. The major research thrusts in opticalinterconnection are in the backplane and board level where theinterconnection distance, the associated parasitic RLC effects, thelarge fan-out induced impedance mismatch jeopardize the bandwidthrequirements, and interference, such as crosstalk, skew and reflection.Optical interconnection has been widely agreed as a better alternativeto upgrade the system performance. For these reasons, a conductive layerhaving relatively high impedance can be replaced by an opticalwaveguide, which can transmit signals at the speed of light. Waveguidesare particularly beneficial when transmitting high speed signals overrelatively long distances, as signal loss is minimized.

While embedded waveguides may be formed in a board or semiconductorsubstrate, difficulties arise when converting electrical signalsemanating from an integrated circuit, mounted on the board's surface, tooptical signals within the embedded waveguide. Some conventionalconversion schemes employ light emitting lasers as transmitters andphoto-detectors as receivers, mounted on the upper surface of a boardadjacent bonding pads/sockets, which receive integrated circuit pins.The electrical signal from an output pin of an integrated circuit istransmitted, via a conductor at or above the board's surface, to thelight emanating laser; which then converts the electrical signal tooptical energy. That optical energy permeates from the board surface,through several layers of the board, downward to a waveguide embeddedwithin the board. A grating coupler is typically placed within thewaveguide to receive the optical energy and directionally transmit anappropriate wave through the waveguide; eventually to be received by anoptical receiver distally located from the grating. An optical receivercan be placed proximate to another integrated circuit, separate from theintegrated circuit initiating the transmitted optical signal. Theoptical receiver can then receive the optical energy, converting it toan electrical signal to be transmitted to an input pin of the receivingintegrated circuit.

Thus, using an optical waveguide enhances the speed at which signals canbe transmitted between integrated circuits. However, inefficiencies intransmitting optical energy through several layers of conductive andnon-conductive materials within a board limit the light-to-electricaland electrical-to-light (optoelectronic) coupling efficiency; therebylimiting high-speed signal transmission within a system.

Additionally, conventional optoelectronic interconnect systems aretypically incompatible with commercial manufacturing processes utilizingboards. Consider, for example, a printed circuit board used as amotherboard within a personal computer. A motherboard manufacturer willtypically, if not exclusively, use automated equipment and processes tomount desired semiconductor devices on the surface of a printed circuitboard. Optoelectronic devices often require care in handling andprocessing that standard semiconductor devices do not. Therefore, use ofconventional optoelectronic interconnect systems will either requiremodification of standard manufacturing processes or additionalprocessing steps to account for the presence or addition ofoptoelectronic components on the board surface. Additional monetary andtime costs resulting from use of conventional optoelectronicinterconnect systems thus render these approaches commercially unviable

Moreover, semiconductor lasers dissipate a lot of electrical power, sogenerated heat can cause catastrophic failure of the laser devicewithout proper cooling. The embedded lasers are also thermally isolatedby surrounding insulators, so heat builds up and the operatingtemperature increases. In addition, an embedded laser cannot be replacedor repaired in a fully embedded integration. As a result, proper thermalmanagement of the laser is pivotal. Present technologies attempt tosolve this problem by using a thermal conductive heat sink assembled ontop of the printed circuit board to cool down the semiconductor laser.These heat sinks are bulky and occupy real estate of the printed circuitboard, which makes alignment to the optical medium, such as an opticalwaveguide, difficult.

As described in U.S. Pat. No. 6,243,509 issued on Jun. 5, 2001, fullyembedded PCB level optical interconnects make the packaging reliable androbust. It provides not only process compatibility with a standard PCBprocess but also reduced footprint of PCB through fully embedding alloptical components such as light sources, channel waveguides anddetectors among other electrical interconnection layers. However, inthis configuration, VCSEL (Vertical Cavity Surface Emitting Laser) arrayas a light source encounters a thermal management concern for the activeregion of the VCSEL arrays because it is encapsulated with thermalinsulators such as polymer waveguide and bonding film (prepreg). Onlythe common bottom metal contact of the VCSEL array can be used as athermal interface. The VCSEL cannot operate without proper cooling.Therefore heat management of driving such a VCSEL array is a criticalissue in the fully embedded structure.

Another issue regarding electrical-to-optical transmitters disposedwithin a printed circuit board involves the fabrication of thereflective elements. The reflective elements are 45 degree waveguidemicro-mirror couplers used to couple light into and out of thewaveguides at 90 degrees. For example, the angle of the plane of theoptical waveguide and the propagation direction of the light source is90 degrees. The reflective elements or 45 degree waveguides aretypically fabricated using laser ablation, oblique reactive ion etching(RIE), temperature controlled RIE, re-flow and machining. The laserablation method is a slow process that is not suitable for thefabrication of a large number of micro-mirros. In addition, it issubjected to lower throughput and surface damage (does not leave asmooth surface, which causes scattering losses). The oblique RIE methodis limited by directional freedom, so it cannot be used if the layout iscomplex (e.g., different direction of micro-mirrors). The temperaturecontrolled RIE method is free from directional freedom but the qualityof the mirror depends on process and materials. The re-flow method isalso subjected to lower throughput. The machining provides good surfaceprofile; however, it is difficult to cut individual waveguide on asubstrate due to the physical size of the machining tool.

Yet another issue regarding electrical-to-optical transmitters disposedwithin a printed circuit board involves the fabrication of the channelwaveguide structure. Typically, the channel waveguide structure isfabricated using photolithography, reactive ion etching, laser ablation,imprinting or molding. The reactive ion etch (RIE) uses ionized gas toremove material where it is not protected by a mask material in a vacuumchamber. The size of the substrate purely depends on the vacuum chamber.It is relatively free from material selection because RIE is a physicalremoving process. The lithography uses optically transparent andphotosensitive materials. Exposed or unexposed area by UV light makesthe material insoluble to solvent due to the cross linking of molecule.However, there is a limitation for choosing material due to the lack ofmaterials which have optical transparency in the interested region andphotosensitivity. Hot embossing and molding are indirect fabricationtechniques by means of transferring waveguide structure on thesubstrate. Embossing plate or cast is first fabricated using the masterwaveguide pattern. Once the plate or the cast was fabricated, the restof processes are purely replication steps. Therefore, these fabricationtechniques are suitable for mass production like stamping of compactdisk. Laser ablation technique is similar to carving without a usingchisel. Highly intensive UV laser beam removes the material of unwantedregion. The motion stage which holds waveguide substrate is moved alongthe predefined paths. Therefore, processing time is quite long. It is aquite versatile tool for small quantities in fabrication and does notrequire a mask pattern. All of these processes, except for imprintingand molding have a slow process time and are not suitable for makinglarge format optical components, such as waveguides or couplers or foruse in mass production lines. The imprinting method can be used to makelarge scale optical waveguide layers, but the fabrication process iscomplex.

There is, therefore, a need for a system, method and apparatus forimproved electrical-to-optical transmitters disposed within a printedcircuit board using improved heat dissipation and fabricationtechniques.

SUMMARY OF THE INVENTION

The present invention uses inventive heat dissipation and fabricationtechniques to improve electrical-to-optical transmitters disposed withina printed circuit board. First, the present invention provides aneffective heat sink embedded within the electro-optical hybrid printedcircuit board using a simple and reliable fabrication process that doesnot require additional steps. Second, the present invention provides animproved method to fabricate the reflective element disposed within thechannel waveguide structure. Third, the present invention provides animproved method to fabricate the channel waveguide structure.

With respect to the heat sink, the present invention uses a thermalconductive material deposited underneath the thin film lasers that arefully embedded inside the PCB. The thermal conductive material can becopper, thermal conductive paste, combination of copper and paste orother suitable material. For example, a heat sink comprising a very thindirectly elecro-deposited copper film can be fabricated using standardelectrical copper plating processes which are commonly used to fabricatethick copper electrical traces. Although thermal conductive pastes canbe used, copper film, which is a highly thermal conductive material,directly deposited on the devices provides an ideal heat sink. Heatdissipation using heat sinks in accordance with the present inventionare significant. The heat sink is fabricated by electroplating, which isa standard process for PCB to fabricate thick electrical traces. Inaddition, this process reduces manufacturing costs by simplifying theassembly process. In spite of the importance of dissipating heat,effective heat dissipation is very difficult to realize, especially infully embedded board level optical interconnects due to the fact thatall optical components are buried between thermally insulated materials,such as interlayer dielectric material. Thermal conductive material,such as copper pillars and copper thin film, is used to make effectiveheat sink underneath the vertical cavity surface emitting lasers for thefully embedded structure that is crucial for next generation high speedboard level interconnects.

The present invention provides a heat sink for an electrical-to-opticaltransmitter disposed within a cavity of a printed circuit board. Theheat sink is a thermal conductive material disposed within the cavityand is thermally coupled to a bottom surface of theelectrical-to-optical transmitter. A portion of the thermal conductivematerial extends approximately to an outer surface of a layer of theprinted circuit board.

The heat sink may be implemented as part of a planarized signalcommunications system that includes a first index buffer layer withinthe printed circuit board, a second index buffer layer within theprinted circuit board and a polymer waveguide disposed below the firstand above the second index buffer layers. The electrical-to-opticaltransmitter is disposed within the first index buffer layer, adjoiningthe polymer waveguide. A reflective element is disposed within thepolymer waveguide in direct alignment with the electrical-to-opticaltransmitter and is adapted to reflect optical energy from theelectrical-to-optical transmitter along the polymer waveguide. Anoptical-to-electrical receiver is disposed within the first index bufferlayer adjoining the polymer waveguide. A reflective element is disposedwithin the polymer waveguide in direct alignment with theoptical-to-electrical receiver and is adapted to reflect optical energyfrom within the polymer waveguide to the optical-to-electrical receiver.An at least partially metal layer is disposed within the printed circuitboard that is fabricated to provide electrical coupling between theelectrical-to-optical transmitter and a surface of the printed circuitboard, and between the optical-to-electrical receiver and the surface ofthe printed circuit board.

In addition, the heat sink may be implemented as part of anoptoelectronic signal communications system that includes a substratehaving a first surface and a waveguide structure fabricated within thesubstrate. The electrical-to-optical transmitter is fabricated withinthe substrate and in direct adjoinment with the waveguide structure.Similarly, a first reflective element is fabricated within the waveguidestructure in direct alignment with the electrical-to-opticaltransmitter. An optical-to-electrical receiver is fabricated within thesubstrate and in direct adjoinment with the waveguide structure and asecond reflective element fabricated within the waveguide structure indirect alignment with the optical-to-electrical receiver. An at leastpartially metal layer is disposed within the substrate and is fabricatedto provide electrical coupling between the electrical-to-opticaltransmitter and the first surface, and between the optical-to-electricalreceiver and the first surface.

The present invention also provides a printed circuit board thatincludes an electrical-to-optical transmitter disposed within a cavityof the printed circuit board and a heat sink disposed within the cavitywherein the heat sink is thermally coupled to a bottom surface of theelectrical-to-optical transmitter and at least a portion of the heatsink extends approximately to an outer surface of a layer of the printedcircuit board. A cooling device may also be thermally coupled to theheat sink. The printed circuit board may comprise a planarized signalcommunications system or an optoelectronic signal communications system.

In addition, the present invention provides a method for fabricating aheat sink for an electrical-to-optical transmitter disposed within acavity of a printed circuit board. The electrical-to-optical transmitterdisposed within a cavity of the printed circuit board is fabricated. Athermal conductive material is then deposited within the cavity whereinthe thermal conductive material is thermally coupled to a bottom surfaceof the electrical-to-optical transmitter and at least a portion of thethermal conductive material extends approximately to an outer surface ofa layer of the printed circuit board.

The present invention also provides an improved method to fabricate thereflective element disposed within the channel waveguide structure. Thereflective element or micro-mirror coupler is formed by cutting thepolymeric waveguide at a 45 degree angle using a very sharp blade. Thefabrication of the reflective element using a motion stage controlled bycomputer is fast and easy. This method is faster and less complicatedthat using laser ablation or oblique reactive ion etching.

Moreover, the present invention provides an improved method to fabricatethe channel waveguide structure. The channel waveguide structure isfabricated using a compression molding technique using UV/thermal crosslinkable polymers. This method enables the simultaneous fabrication ofhigh quality waveguides and couplers, which reduces process time and thenumber of required steps. In addition, this method can be used tofabricate large format optical interconnection layers and opticaltransmitter and/or receiver connectors. This method is better than theother available fabrication methods, such as photolithography, reactiveion etching, laser ablation, and imprinting.

The present invention also provides an optical waveguide circuit havinga flexible optical waveguide film, an electrical-to-optical transmitterconnected to the flexible optical waveguide film, a photoelectricdetector connected to the flexible optical waveguide film, and a firstand second reflective element to optically couple theelectrical-to-optical transmitter to the photoelectric detector via theflexible optical waveguide film. The flexible optical waveguide film canbe SU-8 photoresist. The first and second reflective elements can bewaveguide mirror couplers fabricated using a microtome blade. Thewaveguide mirror couplers can have an angle of 45 degrees plus or minus1.5 degrees. The optical waveguide circuit can have a couplingefficiency that is greater than 90%.

In addition, the present invention provides a method for fabricating anoptical waveguide circuit by providing a waveguide layer, laminating thetop of the waveguide film with a metallic foil, patterning the metallicfoil to form top electrical pads for one or more electrical-to-opticaltransmitters, forming micro vias and bonding devices on the waveguidelayer, and electroplating a metallic film on the back side of theelectrical-to-optical transmitters. The waveguide layer can befabricated using a soft molding process.

Other features and advantages of the present invention will be apparentto those of ordinary skill in the art upon reference to the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the invention may be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which:

FIG. 1 is a cross-sectional view of one embodiment in accordance withthe present invention;

FIG. 2 is a cross-sectional view of another embodiment in accordancewith the present invention;

FIG. 3 is a cross-sectional view of one embodiment of the presentinvention disposed within a PCB;

FIG. 4 is a cross-sectional view of another embodiment of the presentinvention disposed within a PCB;

FIGS. 5A, 5B, 5C, 5D, 5E and 5F are cross-sectional views illustratingone fabrication process in accordance with the present invention;

FIG. 6 illustrates fully embedded PCB level optical interconnects inaccordance with the present invention;

FIGS. 7A, 7B and 7C illustrate the four parts of a VCSEL array, a SEMpicture of 10 μm thick VCSEL, and an enlarged view of a VCSEL inaccordance with the present invention;

FIG. 8 is a graph displaying the Light-Current characteristics as afunction of device thickness in accordance with the present invention;

FIG. 9 is a graph displaying wavelength shift as a function of netdissipated power in accordance with the present invention;

FIG. 10 is a cross sectional diagram of a VCSEL cooling structure of 250μm thick copper block or thermal paste in accordance with the presentinvention;

FIG. 11 is a cross sectional diagram of a VCSEL cooling structure of 30μm thick electroplated copper film in accordance with the presentinvention;

FIG. 12A illustrates a 2D finite element analysis results for agenerated mesh in accordance with the present invention;

FIG. 12B illustrates a 2D finite element analysis results for a 250 μmthick copper block, 250 μm thick VCSEL, θ_(jc)=39.4° C. in accordancewith the present invention;

FIG. 12C illustrates a 2D finite element analysis for a 250 μm thermalconductive paste, 250 μm thick VCSEL, θ_(jc)=45.9° C. in accordance withthe present invention;

FIG. 13 depicts measured device thermal resistances as a function ofdevice thickness in accordance with the present invention;

FIG. 14 depicts calculated thermal resistances as a function of devicethickness for buried VCSEL with 30 μm thick electroplated Cu film heatsink in accordance with the present invention;

FIG. 15A is a SEM photograph of the waveguide structures with 45°waveguide mirrors and FIG. 15B is an enlarged view of the mirror surfacein accordance with the present invention;

FIG. 16 is a graph of coupling efficiencies as a function of angulardeviation from 45° for 127 μm thick substrate with 12 μm aperture VCSELin accordance with the present invention;

FIGS. 17A and 17B are cross sectional views of SU-8 photoresist patternfor various exposure conditions (Exposure: 300 mJ/cm2) with a UV-34filter and a UV-34 filter and Index matching oil in accordance with thepresent invention;

FIG. 18 is a photograph of a PDMS Waveguide mold in accordance with thepresent invention;

FIG. 19 illustrates a flexible optical waveguide film fabricationprocess flow in accordance with the present invention;

FIG. 20 is a photograph of a fabricated flexible optical waveguide filmin accordance with the present invention;

FIG. 21 is a graph of extracted absorption loss of the Topas™ 5013 as afunction of wavelength in accordance with the present invention;

FIG. 22 is a graph of coupled out power as a function of waveguidelength in accordance with the present invention;

FIG. 23 shows the coupled out beams from 45° waveguide mirrors; inaccordance with the present invention;

FIG. 24 illustrates a device integration process flow chart inaccordance with the present invention;

FIG. 25 shows a flexible optical waveguide film, 12-channel VCSEL array,12-channel PIN photodiode array and 45° micro-mirror couplers inaccordance with the present invention;

FIG. 26 is a graph showing the L-I characteristics of a 12-channel VCSELarray for the 10 Gb/s and the 2.5 Gb/s in accordance with the presentinvention; and

FIG. 27 illustrates an integrated VCSEL and detector arrays on aflexible optical waveguide film in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts thatcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and do not delimit the scope of theinvention.

The present invention uses inventive heat dissipation and fabricationtechniques to improve electrical-to-optical transmitters disposed withina printed circuit board. First, the present invention provides aneffective heat sink embedded within the electro-optical hybrid printedcircuit board using a simple and reliable fabrication process that doesnot require additional steps. Second, the present invention provides animproved method to fabricate the reflective element disposed within thechannel waveguide structure. Third, the present invention provides animproved method to fabricate the channel waveguide structure.

With respect to the heat sink, the present invention uses a thermalconductive material deposited underneath the thin film lasers that arefully embedded inside the PCB. The thermal conductive material can becopper, thermal conductive paste, combination of copper and paste orother suitable material. For example, a heat sink comprising a very thindirectly elecro-deposited copper film can be fabricated using standardelectrical copper plating processes which are commonly used to fabricatethick copper electrical traces. Although thermal conductive pastes canbe used, copper film, which is a highly thermal conductive material,directly deposited on the devices provides an ideal heat sink. Heatdissipation using heat sinks in accordance with the present inventionare significant. The heat sink is fabricated by electroplating, which isa standard process for PCB to fabricate thick electrical traces. Inaddition, this process reduces manufacturing costs by simplifying theassembly process. In spite of the importance of dissipating heat,effective heat dissipation is very difficult to realize, especially infully embedded board level optical interconnects due to the fact thatall optical components are buried between thermally insulated materials,such as interlayer dielectric material. Thermal conductive material,such as copper pillars and copper thin film, is used to make effectiveheat sink underneath the vertical cavity surface emitting lasers for thefully embedded structure that is crucial for next generation high speedboard level interconnects.

The present invention also provides an improved method to fabricate thereflective element disposed within the channel waveguide structure. Thereflective element or micro-mirror coupler is formed by cutting thepolymeric waveguide at a 45 degree angle using a very sharp blade. Thefabrication of the reflective element using a motion stage controlled bycomputer is fast and easy. This method is faster and less complicatedthat using laser ablation or oblique reactive ion etching.

Moreover, the present invention provides an improved method to fabricatethe channel waveguide structure. The channel waveguide structure isfabricated using a compression molding technique using UV/thermal crosslinkable polymers. This method enables the simultaneous fabrication ofhigh quality waveguides and couplers, which reduces process time and thenumber of required steps. In addition, this method can be used tofabricate large format optical interconnection layers and opticaltransmitter and/or receiver connectors. This method is better than theother available fabrication methods, such as photolithography, reactiveion etching, laser ablation, and imprinting.

Now referring to FIG. 1, a cross-sectional view of one embodiment inaccordance with the present invention is shown. An electrical-to-opticaltransmitter 100 is disposed within a cavity 102 of a printed circuitboard 104. The electrical-to-optical transmitter 100 has an aperture 106and terminal pads 108. The electrical-to-optical transmitter 100 can bea laser, vertical cavity surface emitting laser (VCSEL) or an edgeemitting laser. A heat sink 110 comprising a thermal conductive materialis disposed within the cavity 102 wherein the thermal conductivematerial is thermally coupled to a bottom surface 112 of theelectrical-to-optical transmitter 100 and at least a portion of thethermal conductive material extends approximately to an outer surface(120, 122, 124, etc.) of a layer (114, 116, 118, etc.) of the printedcircuit board 104. As a result, the heat sink 110 may extend to surfaces126, 128 or 130. Note that the dashed lines indicate that the heat sink110 can occupy one or more layers of the printed circuit board and isnot limited to a maximum of three layers as illustrated. In addition,the heat sink 110 can extend only partially into the next layer,although fabrication of such a heat sink 110 will be more difficult andcostly.

The thermal conductive material can be copper, thermal conductive paste,combination of copper and a paste, or any other suitable material. Asshown, the thermal conductive material substantially fills the cavity102 from the bottom of the electrical-to-optical transmitter 100 toapproximately the outer surface 126, 128 or 130 of a layer of theprinted circuit board 114, 116 or 118. In typical applications, theelectrical-to-optical transmitter 100 is between 10 and 250 μm inthickness and the heat sink 110 is between 10 and 490 μm in thickness.Note that a cooling device may be thermally coupled to a portion of theouter surface 126, 128 or 130 of the heat sink 110.

Referring now to FIG. 2, a cross-sectional view of another embodiment inaccordance with the present invention is shown. An electrical-to-opticaltransmitter 100 is disposed within a cavity 102 of a printed circuitboard 104. The electrical-to-optical transmitter 100 has an aperture 106and terminal pads 108. The electrical-to-optical transmitter 100 can bea laser, vertical cavity surface emitting laser (VCSEL) or an edgeemitting laser. A heat sink 200 comprising a thermal conductive materialis disposed within the cavity 102 wherein the thermal conductivematerial is thermally coupled to a bottom surface 112 of theelectrical-to-optical transmitter 100 and at least a portion of thethermal conductive material extends approximately to an outer surface(120, 122, 124, etc.) of a layer (114, 116, 118, etc.) of the printedcircuit board 104. As a result, the heat sink 200 may extend to surfaces202, 204 or 206. Note that the dashed lines indicate that the heat sink200 can occupy one or more layers of the printed circuit board and isnot limited to a maximum of three layers as illustrated. In addition,the heat sink 200 can extend only partially into the next layer,although fabrication of such a heat sink 200 will be more difficult andcostly.

The thermal conductive material can be copper, thermal conductive paste,combination of copper and a paste, or any other suitable material. Asshown, the thermal conductive material comprises a film on the bottomsurface 112 of the electrical-to-optical transmitter and an interiorwall 208 of the cavity 102 extending approximately to the outer surface126, 128 or 130 of a layer of the printed circuit board 114, 116 or 118.In typical applications, the electrical-to-optical transmitter 100 isbetween 10 and 250 μm in thickness and the film is approximately isapproximately 10 to 50 μm in thickness (30 μm is typical). Note that acooling device may be thermally coupled to a portion of the outersurface 202, 204 or 206 of the heat sink 200.

Now referring to FIG. 3, a cross-sectional view of one embodiment of thepresent invention disposed within a PCB is shown. Anelectrical-to-optical transmitter 100 is disposed within a cavity 102 ofa printed circuit board 300. The electrical-to-optical transmitter 100can be a laser, vertical cavity surface emitting laser (VCSEL) or anedge emitting laser. A heat sink 110 comprising a thermal conductivematerial is disposed within the cavity 102 wherein the thermalconductive material is thermally coupled to a bottom surface 112 of theelectrical-to-optical transmitter 100 and at least a portion of thethermal conductive material extends approximately to an outer surface(120, 122, 124, etc.) of a layer (114, 116, 118, etc.) of the printedcircuit board 300. As a result, the heat sink 110 may extend to surfaces126, 128 or 130. Note that the dashed lines indicate that the heat sink110 can occupy one or more layers of the printed circuit board and isnot limited to a maximum of three layers as illustrated. In addition,the heat sink 110 can extend only partially into the next layer,although fabrication of such a heat sink 110 will be more difficult andcostly.

The thermal conductive material can be copper, thermal conductive paste,combination of copper and a paste, or any other suitable material. Asshown, the thermal conductive material substantially fills the cavity102 from the bottom of the electrical-to-optical transmitter 100 toapproximately the outer surface 126, 128 or 130 of a layer of theprinted circuit board 114, 116 or 118. In typical applications, theelectrical-to-optical transmitter 100 is between 10 and 250 μm inthickness and the heat sink 110 is between 10 and 490 μm in thickness.Note that a cooling device may be thermally coupled to a portion of theouter surface 126, 128 or 130 of the heat sink 110.

A board level optical interconnect is formed by forming a channelwaveguide structure 302 above a lower cladding layer 304 in which theelectrical-to-optical transmitter 100 is embedded. An upper claddinglayer 306 is disposed above the channel waveguide structure 302. One ormore additional layers 308, 310 and 312 may be disposed above the uppercladding layer 306. A reflective element 314 is disposed within thechannel waveguide structure 302 in direct alignment with theelectrical-to-optical transmitter 100. Metal structures or vias (notshown) will be connected to the terminals of the electrical-to-opticaltransmitter 100.

The reflective elements 314 are 45 degree waveguide micro-mirrorcouplers used to couple light into and out of the channel waveguidestructure 302 at 90 degrees. For example, the angle of the plane of theoptical waveguide 302 and the propagation direction of the light source(optical transmitter 100) is 90 degrees. Various methods can be used tofabricate the reflective element 314 disposed within the channelwaveguide structure 302, such as oblique reactive ion etching or laserablation. The present invention uses an improved method wherein thereflective element 314 or micro-mirror coupler is formed by cutting thepolymeric waveguide 302 at a 45 degree angle using a very sharp blade.The fabrication of the reflective element 314 using a motion stagecontrolled by computer is fast and easy. This method is faster and lesscomplicated that using laser ablation or oblique reactive ion etching.This method will be described in more detail below.

Likewise, the channel waveguide structure 302 can be fabricated usingvarious techniques, such as photolithography, reactive ion etching,laser ablation, imprinting or molding. The present invention uses animproved method wherein the channel waveguide structure 302 isfabricated using a compression molding technique using UV/thermal crosslinkable polymers. This method enables the simultaneous fabrication ofhigh quality waveguides 302 and couplers 314, which reduces process timeand the number of required steps. In addition, this method can be usedto fabricate large format optical interconnection layers and opticaltransmitter and/or receiver connectors. This method will be described inmore detail below.

Referring now to FIG. 4, a cross-sectional view of another embodiment ofthe present invention disposed within a PCB is shown. Anelectrical-to-optical transmitter 100 is disposed within a cavity 102 ofa printed circuit board 400. The electrical-to-optical transmitter 100can be a laser, vertical cavity surface emitting laser (VCSEL) or anedge emitting laser. A heat sink 200 comprising a thermal conductivematerial is disposed within the cavity 102 wherein the thermalconductive material is thermally coupled to a bottom surface 112 of theelectrical-to-optical transmitter 100 and at least a portion of thethermal conductive material extends approximately to an outer surface(120, 122, 124, etc.) of a layer (114, 116, 118, etc.) of the printedcircuit board 104. As a result, the heat sink 200 may extend to surfaces202, 204 or 206. Note that the dashed lines indicate that the heat sink200 can occupy one or more layers of the printed circuit board and isnot limited to a maximum of three layers as illustrated. In addition,the heat sink 200 can extend only partially into the next layer,although fabrication of such a heat sink 200 will be more difficult andcostly.

The thermal conductive material can be copper, thermal conductive paste,combination of copper and a paste, or any other suitable material. Asshown, the thermal conductive material comprises a film on the bottomsurface 112 of the electrical-to-optical transmitter and an interiorwall 208 of the cavity 102 extending approximately to the outer surface126, 128 or 130 of a layer of the printed circuit board 114, 116 or 118.In typical applications, the electrical-to-optical transmitter 100 isbetween 10 and 250 μm in thickness and the film is approximately isapproximately 10 to 50 μm in thickness (30 μm is typical). Note that acooling device may be thermally coupled to a portion of the outersurface 202, 204 or 206 of the heat sink 200.

A board level optical interconnect is formed by forming a channelwaveguide structure 402 above a lower cladding layer 404 in which theelectrical-to-optical transmitter 100 is embedded. An upper claddinglayer 406 is disposed above the channel waveguide structure 402. One ormore additional layers 408, 410 and 412 may be disposed above the uppercladding layer 406. A reflective element 414 is disposed within thechannel waveguide structure 402 in direct alignment with theelectrical-to-optical transmitter 100. Metal structures or vias (notshown) will be connected to the terminals of the electrical-to-opticaltransmitter 100.

The reflective elements 414 are 45 degree waveguide micro-mirrorcouplers used to couple light into and out of the channel waveguidestructure 402 at 90 degrees. For example, the angle of the plane of theoptical waveguide 402 and the propagation direction of the light source(optical transmitter 100) is 90 degrees. Various methods can be used tofabricate the reflective element 414 disposed within the channelwaveguide structure 402, such as oblique reactive ion etching or laserablation. The present invention uses an improved method wherein thereflective element 414 or micro-mirror coupler is formed by cutting thepolymeric waveguide 402 at a 45 degree angle using a very sharp blade.The fabrication of the reflective element 414 using a motion stagecontrolled by computer is fast and easy. This method is faster and lesscomplicated that using laser ablation or oblique reactive ion etching.This method will be described in more detail below.

Likewise, the channel waveguide structure 402 can be fabricated usingvarious techniques, such as photolithography, reactive ion etching,laser ablation, imprinting or molding. The present invention uses animproved method wherein the channel waveguide structure 402 isfabricated using a compression molding technique using UV/thermal crosslinkable polymers. This method enables the simultaneous fabrication ofhigh quality waveguides 402 and couplers 414, which reduces process timeand the number of required steps. In addition, this method can be usedto fabricate large format optical interconnection layers and opticaltransmitter and/or receiver connectors. This method will be described inmore detail below.

Now referring to FIGS. 5A, 5B, 5C, 5D, 5E and 5F, cross-sectional viewsillustrating one fabrication process in accordance with the presentinvention are shown. The system described is one embodiment of anoptoelectronic signal communications system according to the presentinvention using a multi-layer printed circuit board whereinsemiconductor devices can be mounted on its external surfaces. A planar,optoelectrical communications system is embedded within the board,integrated amongst various metallization and dielectric layers. Morespecifically, an electrical-to-optical transmitter or VCSEL 500 having an-contact pad 502 is disposed on top of PWB layer 504. Depending ondesired wavelength, accuracy, and other design considerations, othersuitable electrical-to-optical conversion devices, such as edge emittinglasers, may be employed. Using VCSELs, however, a modulation bandwidthas high as 6 GHZ may be realized at a wavelength of 850 nm.Additionally, such a wavelength is compatible with silicon basedphoto-detectors. The VCSEL 500 may be fabricated using thin-filmprocesses known in the art (e.g. epitaxial lift off). Most importantly,VCSEL 500 is fabricated using processes compatible with those used tofabricate the printed circuit board as the board is being fabricated.Cavity 506 is formed to accommodate a heat sink or thermal via inaccordance with the present invention.

Lower cladding layer 508 (also referred to as an index buffer layer) andvia 510 to establish electrical contact to the p-contact pads of theVCSEL 500 are formed in FIG. 5B. A channel waveguide structure 512 isformed on the lower cladding layer 508 and includes reflective elementcavities 514 and 516 in FIG. 5C. In a preferred embodiment, waveguide512 is a polyimide-based waveguide. Such a polyimide waveguide may befabricated on different substrates such as PC board silicon, glass, andothers by spin coating. For example, an A-600 primer layer may be spincoated first on a substrate with a spin speed of 5000 rpm, and prebakedat 90° C. for 60 seconds. Next, a polyimide, such as Amoco Polyimide9120D, is spin coated with a speed of 2000 RPM. A final curing at 260°C. in nitrogen atmosphere for more than 3 hours is completed. Typicalthickness of a resulting waveguide is 7 μm. Planar waveguides have alsobeen fabricated on silicon substrate by inserting a high index polyimidelayer (N=1.56 to 1.76) between 90/20D cladding layers.

In a preferred embodiment, present invention uses an improved methodwherein the channel waveguide structure 512 is fabricated using acompression molding technique using UV/thermal cross linkable polymers.This method enables the simultaneous fabrication of high qualitywaveguides and couplers, which reduces process time and the number ofrequired steps. In addition, this method can be used to fabricate largeformat optical interconnection layers and optical transmitter and/orreceiver connectors.

The reflective elements 518 and 520 are formed in the reflective elementcavities 514 and 516, respectively, while maintaining via 510 in FIG.5D. Reflective elements 518 and 520 may comprise any suitable structureadaptable to provide efficient optoelectronic coupling, capable ofco-fabrication with waveguide 512. Depending on desired performance andmanufacturing parameters, mirror structures may be utilized. In apreferred embodiment, however, tilted waveguide gratings are employed toprovide a 1 to 1 surface normal coupling. In this embodiment, both thewaveguide(s) and the tilted gratings are polyimide based, to providehigh temperature processing associated with silicon process fabrication.More specifically, the embodiment incorporates a titled grating profilein a planar structure within a thin waveguide layer upon which othermicro-lithographically defined electrical interconnection layers can bebuilt. Such a configuration requires the insertion of opticalinterconnect layer to be planarized. The tilted grating profile greatlyenhances coupling efficiency in the desired direction. Such gratings maybe fabricated with known micro-fabrication processes; rendering themmass-producible with excellent accuracy and reproducibility. As anexample, such a grating coupler may be fabricated byreactive-ion-etching (RIE).

In a preferred embodiment, the present invention uses an improved methodwherein the reflective element 518 and 520 or micro-mirror coupler isformed by cutting the polymeric waveguide 512 at a 45 degree angle usinga very sharp blade. The fabrication of the reflective element 518 and520 using a motion stage controlled by computer is fast and easy. Thismethod is faster and less complicated that using laser ablation oroblique reactive ion etching. This method will be described in moredetail below.

An optical-to-electrical receiver or silicon-based photodetector 522having contact pad 524 is also fabricated. As such, photodectector 522may be fabricated by a silicon metal-semiconductor-metal (MSM) process.Again, depending on desired performance characteristics and productionparameters, other suitable optical-to-electrical conversion devices maybe used. Photodetector 522 is fabricated using processes compatible withthose used to fabricate the board. Upper cladding layer 526 is disposedabove channel waveguide structure 512 and maintains via 510 in FIG. 5E.Via 528 is also formed to provide electrical contact to thephotodetector contact pad 524. The VCSEL 500 generates optical energy tobe generated directly into waveguide 512. Similarly photodectector 522allows optical energy to be drawn directly from waveguide 512.Reflective elements 518 and 520 are formed within waveguide 512 inalignment with elements 500 and 522, to provide direction of transmittedoptical energy. The present invention thus provides highly efficientoptoelectronic coupling.

In FIG. 5F, adhesive layer 530 is disposed above the upper claddinglayer 526. In addition, one or more PWB layers, such as 532 and 534 aredisposed above the adhesive layer 530. Various copper traces 536, 538,540, 542, 544, 546 and 548 are created at various levels as dictated bythe applicable design requirements. Copper trace 536 is connected to theVCSEL 500 by via 510. Copper trace 542 is connected to trace 544 by via550 and trace 544 is connected to trace 538 by via 552. Copper trace 546is connected to copper trace 548 by via 554. Copper trace 540 isconnected to contact pad 524 by via 556. Similarly, adhesive layer 558is disposed below the PWB layer 504 with one or more PWB layers, such as560 and 562 disposed below adhesive layer 558. Cavity 506 is filled witha thermal conductive material as shown in FIGS. 1 or 2 in order to formheat sink 564, which may be connected to a copper trace or additionalheat sink 566. Various copper traces 568, 570, 572, 574, 576 and 578 arecreated at various levels as dictated by the applicable designrequirements. Copper trace 568 is connected to trace 570 by via 580.Copper trace 572 is connected to copper trace 574 by via 582 and coppertrace 574 is connected to copper trace 576 by via 584. The vias andcopper traces can be used to provide direct coupling between theembedded optoelectronic elements and the board surface, which providesthe capability to utilize a variety of integrated circuit devicesincluding a surface mount device or standard pin mount device. Thus,integrated circuit devices may be mounted on the board using standardassembly and manufacturing processes. The presence of the optoelectroniccomponents is realized only in higher performance; that presenceotherwise being transparent to the user of the board.

Based on the foregoing description, it is apparent that the heat sinkmay be implemented as part of a planarized signal communications systemthat includes a first index buffer layer within the printed circuitboard, a second index buffer layer within the printed circuit board anda polymer waveguide disposed below the first and above the second indexbuffer layers. The electrical-to-optical transmitter is disposed withinthe first index buffer layer, adjoining the polymer waveguide. Areflective element is disposed within the polymer waveguide in directalignment with the electrical-to-optical transmitter and is adapted toreflect optical energy from the electrical-to-optical transmitter alongthe polymer waveguide. An optical-to-electrical receiver is disposedwithin the first index buffer layer adjoining the polymer waveguide. Areflective element is disposed within the polymer waveguide in directalignment with the optical-to-electrical receiver and is adapted toreflect optical energy from within the polymer waveguide to theoptical-to-electrical receiver. An at least partially metal layer isdisposed within the printed circuit board that is fabricated to provideelectrical coupling between the electrical-to-optical transmitter and asurface of the printed circuit board, and between theoptical-to-electrical receiver and the surface of the printed circuitboard.

In addition, the heat sink may be implemented as part of anoptoelectronic signal communications system that includes a substratehaving a first surface and a waveguide structure fabricated within thesubstrate. The electrical-to-optical transmitter is fabricated withinthe substrate and in direct adjoinment with the waveguide structure.Similarly, a first reflective element is fabricated within the waveguidestructure in direct alignment with the electrical-to-opticaltransmitter. An optical-to-electrical receiver is fabricated within thesubstrate and in direct adjoinment with the waveguide structure and asecond reflective element fabricated within the waveguide structure indirect alignment with the optical-to-electrical receiver. An at leastpartially metal layer is disposed within the substrate and is fabricatedto provide electrical coupling between the electrical-to-opticaltransmitter and the first surface, and between the optical-to-electricalreceiver and the first surface.

The present invention also provides a printed circuit board thatincludes an electrical-to-optical transmitter disposed within a cavityof the printed circuit board and a heat sink disposed within the cavitywherein the heat sink is thermally coupled to a bottom surface of theelectrical-to-optical transmitter and at least a portion of the heatsink extends approximately to an outer surface of a layer of the printedcircuit board. A cooling device may also be thermally coupled to theheat sink. The printed circuit board may comprise a planarized signalcommunications system or an optoelectronic signal communications system.

In addition, the present invention provides a method for fabricating aheat sink for an electrical-to-optical transmitter disposed within acavity of a printed circuit board. The electrical-to-optical transmitterdisposed within a cavity of the printed circuit board is fabricated. Athermal conductive material is then deposited within the cavity whereinthe thermal conductive material is thermally coupled to a bottom surfaceof the electrical-to-optical transmitter and at least a portion of thethermal conductive material extends approximately to an outer surface ofa layer of the printed circuit board.

FIG. 6 illustrates fully embedded PCB level optical interconnects inaccordance with the present invention. The fully embedded board-levelguided wave optical interconnection provides high speed opticalcommunications within one board 600. These include a vertical cavitysurface emitting laser (VCSEL) 602, surface-normal waveguide couplers604, and a polymer-based channel waveguide 606 functioning as thephysical layer of optical bus (interconnection) and anoptical-to-electrical receiver or photoreceiver 608. The drivingelectrical signals to modulate the VCSEL 602 and the demodulated signalsreceived at the photoreceiver 608 flow through electrical vias 610connecting to the surface 612 of the PC board 600. The fully embeddedstructure makes the insertion of optoelectronic components intomicroelectronic systems much more realistic when considering the factthat the major stumbling block for implementing optical interconnectiononto high performance microelectronics is the packaging incompatibility.All the real estate of the PCB surface is occupied by electronics not byoptoelectronic components. The performance enhancement due to theemployment of the optical interconnection has been observed without theinterface problems between electronic and optoelectronic components thatconventional approaches have.

Various embodiments of the present invention were fabricated and tested.The following describes this fabrication process and testing as anexample and not as a limitation of the present invention. To providesystem integration using guided wave optical interconnection,polymer-based material has its exclusive advantages. It can bespin-coated on a myriad of substrates with a relatively largeinterconnection distance. To ensure the desired electrical andmechanical properties imposed on board level optical interconnection,and to meet the required optical properties for the low loss waveguideformation, the photosensitive polyimide provided by Amoco Chemicals wasused for the waveguide fabrication. 45° total internal reflection (TIR)mirrors were fabricated by oblique reactive ion etching (RIE) to providesurface normal coupling. The propagation loss of the TE mode of thechannel waveguide is 0.21 dB/cm at 850 nm.

The crosstalk of parallel channel waveguides is important factor incommunication. The channel waveguide pitch is 250 μm and the width ofwaveguide is 50 μm. The refractive difference between core and claddingis about 0.01. To measure crosstalk, the sample was put on auto-alignerand the fiber coupled laser light with wavelength of 630 nm was lunchedinto one channel among the waveguides. The output of the signal wasdetected from the adjacent channel from the input channel. The inputpower at waveguide is −21 dBm and the output power of the adjacentchannel is −53 dBm. The measured crosstalk was 32 dB.

High performance vertical cavity surface emitting lasers (VCSELs) arecommercially available with the output wavelength of 850 nm. These laserdevices, compared to conventional edge emitting lasers, provide a verylow threshold current with much less temperature sensitivity, moderateoptical power (few mW), very high direct modulation bandwidth (>14 GHz),wide operating temperature range (−55 to +125° C.), and ease ofpackaging in an array configuration due to the unique surface-normaloutput nature. Planar configuration of VCSELs allows these devices to befabricated, wafer scale tested with conventional microelectronicsmanufacturing processes. The unique surface-normal emitting nature ofthe device allows us to use exactly the same packaging scheme forcoupling light from VCSEL into waveguide as that used for coupling lightfrom waveguide into photodetector.

All optical components including VCSEL arrays, photo-detector arrays andplanarized waveguide arrays are fully embedded among electrical layersin the board level optical interconnects. As a result, the assemblysteps at final laminating stage are simplified using standard PCBmanufacturing procedure. Generally, 5 to 10 mils (127 to 254 μm) thickcopper laminated polymer layer and 4 mils thick (100 μm) bonding filmsare used in multi layer PCB. For the fully embedded structure, thinVCSEL and photo-detector, both are 10 μm thick, are buried amongelectrical PCB layers. Each of electrical PWB layers 614 functions todistribute electrical signals or power or as grounding layer as shown inFIG. 6. Through-holes and vias 610 are used to transport electricalsignals among electrical PWB layers 614 and also to provide electricalconnections to VCSEL 616 and detector 608 arrays. In the fully embeddedstructure, entire real estates of top and/or bottom sides of the PCB areoccupied only with microelectronic ICs which perform the designatedfunctions including electrical-to-optical and optical-to-electricalconversions through vias 610 connected thin film VCSELs 602 andphoto-detectors 608.

To incorporate the VCSEL array onto the fully embedded architecture, theVCSEL array has to be thin enough to build such a 3-D structure. Anoxide confined thin film linear VCSEL array was fabricated. A fullyembedded board-level guided wave optical interconnection is presented.All elements involved in providing high speed optical communicationswithin one board are demonstrated. These include a thin vertical cavitysurface emitting laser (VCSEL), surface-normal waveguide couplers, apolyimide-based channel waveguide functioning as the physical layer ofoptical bus and a photoreceiver. The driving electrical signal tomodulate the VCSEL and the demodulated signal received at thephotoreceiver can be applied through electrical vias connecting to thesurface of the PC board. The epitaxial structure of the VCSEL array wasgrown on GaAs substrate. An etch stop layer of 100 nm thickAl_(0.98)Ga_(0.02)As was grown and then GaAs buffer layer, 40.5 pairs ofn-DBR, three GaAs quantum wells, and 23 pairs of p-DBR were grown. Totalthickness of epitaxial structure is 10 μm.

Formation of thin film VCSEL arrays started with wet etch to makeannular shape trench which provides isolation of each device and definesoxide confinement region. Wet oxidation was carried out in quartz tubefurnace which was held at 460° C. Spin on glass (SOG) was coated on theentire wafer for electrical isolation and side wall sealing afterwards.The SOG opening process was followed for p-contact metallization. VCSELarray formation was followed by the substrate removal process to formthe required thickness. Devices were first mechanically thinned down to250 μm. These devices were back etched using wet etching to make variousthick VCSELs (200, 150, 100 μm). Ten micrometer thick VCSEL was formed.FIG. 7A shows fabricated 1×12 VCSEL arrays and FIG. 7B shows crosssection view of 10 μm thick VCSEL array after substrate removal. FIG. 7Cillustrates an enlarged view of a VCSEL in accordance with the presentinvention.

FIG. 8 is a graph displaying the Light-Current characteristics as afunction of device thickness in accordance with the present invention.The Light—Current (“L-I”) characteristics of various thinned VCSELs areshown where the quantum efficiency of the 10 μm thick VCSEL is increasedby ˜50% then the driving current is above 9 mA. Threshold current were3.5 mA for all devices without showing any degradation, however slopeefficiency was increased by reducing device thickness due to thereducing device thermal resistance. The substrate removed VCSEL (10 μmthick) shows higher slope efficiency than thicker devices did (FIG. 9).By reducing device thickness, more strate slope efficiency is obtained,even high injection current level.

The substrate removed VCSEL (10 μm thick) also shows linear dependencyeven at high injection current. Thermal resistance was calculated fromthe measured wavelength shift as a function of substrate temperature anddissipation power. The thermal resistance is given byR_(th)=ΔT/ΔP=(Δλ/ΔP)/(Δλ/ΔT), where ΔT is the change of junctiontemperature, ΔP is the change of injected power and Δλ is the wavelengthshift. Both Δλ/ΔP and Δλ/ΔT are experimentally confirmed.

The device under test (DUT) was laid on the top of gallium indiumeutectic metal which is used to make electrical connect. The substratetemperature was controlled by thermo-electric cooler (TEC). The measuredwavelength shift as a function of temperature for all devices was 0.75Å/° C. The wavelength shifts as a function of net dissipated power were0.59, 0.54, 0.5, 0.43, 0.36 Å/mW, respectively, corresponding to 250,200, 150, 100, 10 μm thick VCSELs as indicated in FIG. 9. The thermalresistances for 250, 200, 150, 100, 10 μm thick VCSELs were measured tobe 772, 710, 657, 572, 478° C./W, respectively. Note that the thicknessof the 10 μm thick VCSEL has an exclusive advantage of heat managementdue to the reduction of the thermal resistance shown herein.

As previously discussed, the VCSEL is a major heat source in fullyembedded optical interconnects structure. The embedded VCSEL arrays arethermally isolated by surrounding insulators; therefore heat builds upand the operating temperature increases. High operating temperaturereduces lifetime of device and laser output power. Reliable operation ofthe VCSEL is needed through proper heat management. Effective heatremoval is a challenging task in embedded structure because thepackaging compatibility to PCB manufacturing process has to beconsidered while providing an effective and simple cooling mechanism.

The present invention provides an effective heat management scheme. Twodifferent cooling structures were compared as shown in FIGS. 10 and 11.FIG. 10 illustrates one embodiment of the present invention using a heatsink 1000 comprising a conductive material of 250 μm thick bulk copperor H2OE epoxy. The heat sink 1000 is deposited on the n-contact pad 1002of the VCSEL 1004. The VCSEL 1004 also has an aperture 1006 and topcontact pads 1008. The bottom surface 1010 of the heat sink 1000 copperblock was maintained at 25° C. during the testing. FIG. 11, on the otherhand, illustrates another embodiment of the present invention using aheat sink 1100 comprising a conductive material of 30 μm thickelectrodeposited copper film. 30 μm thick copper film was chosen as theheat sink 1100 because this is the thickness of copper trace inelectrical layer for PCB. The copper film was directly electro-depositedon the n-contact metal pad 1002 of the VCSEL 1004 array duringelectroplating step. The bottom surface 1102 of the heat sink 1100copper block was maintained at 25° C. during testing.

The n-contact metal affiliated with the bottom DBR mirror of the VCSELdie was directly electroplated with copper during process without anyother thermal conductive paste which has lower conductivity than copper.Usually several tens of micrometer thick copper was deposited in coppercontained acid chemical solution during PCB process. It can be used as avery good electrical and thermal passage simultaneously. Thermalresistance of the VCSEL depends on the device structure itself and alsopackaging structure. Direct bonding of a device using electroplatingreduces thermal resistance of device due to the absence of lowconductivity bonding epoxy.

The ANSYS program was used to perform 2-D finite element thermaldistribution analysis. The thermal conductivities of GaAs, DBR mirrorand copper are 4.6×10⁻⁵ W/μm ° K, 2.3×10⁻⁵ W/μm K, and 4×10⁻⁴ W/μm ° K,respectively. Heat is generated due to the Bragg reflector's resistanceand imperfect conversion efficiency in active region. However, the heatgenerated due to the DBR is relatively small compared with activeregion, therefore this term was ignored in simulation. The heatgeneration rate in active region (circular shape, diameter of 18 μm) isbased on measured value which is 20 mW per VCSEL.

The simulation results are shown in FIGS. 12A, 12B and 12C. FIG. 12A isthe generated mesh profile. FIG. 12B illustrates a 2D finite elementanalysis results for a 250 μm thick copper block, 250 μm thick VCSEL,θ_(jc)=39.4° C. FIG. 12C illustrates a 2D finite element analysis for a250 μm thermal conductive paste, 250 μm thick VCSEL, θ_(jc)=45.9° C. For250 μm thick copper heat sink block, the temperature at active regionreached 39.4° C. corresponding to thermal resistance of 722 ° K/W (FIG.12B). For the case of 30 μm thick electrodeposited copper film heatsink, junction temperature reached 34.58° C. as in FIG. 12Ccorresponding to thermal resistance of 455 ° K/W. The higher junctiontemperature reduces quantum efficiency and causes catastrophic failureof the device. Despite of lower thermal resistance of 250 μm thickcopper heat sink block, this can not be used in fully embedded structuredue to difficulty of realization.

The measured and calculated thermal resistances of the devices aresummarized in FIGS. 13 and 14. As shown in FIG. 13, the calculatedthermal resistances of devices are well matched with measured results.According to this result, the simulation model and process were properlycarried out. FIG. 14 shows theoretically determined thermal resistancesfor various thick VCSELs. For 30 μm thick electroplated copper film, thejunction temperatures were theoretically determined to be 43.8, 43,42.2, 41.5, 40.2 and 34.6° C. for 250, 200, 150, 100, 50 and 10 μm thickVCSEL, respectively. The substrate removed VCSEL having a totalthickness of 10 μm shows superior optical and thermal characteristics.The maximum allowable device thickness to meet the requirement ofreliable operation in the fully embedded integration can be determined.

Various fabrication methods for the present invention will now bedescribed in more detail with respect to a flexible optical waveguidefilm with integrated optoelectronic devices (VCSEL and PIN photodiodearrays) for fully embedded board level optical interconnects. Theoptical waveguide circuit is fabricated with 45° micro-mirror couplers(reflective elements) on a thin flexible polymeric substrate (channelwaveguide structure) by soft molding. The 45° couplers are fabricated bycutting waveguide with microtome blade. The waveguide core material isSU-8 photoresist and the cladding is Cycloolefin copolymer (COC). A thinVCSEL and PIN photodiode array are directly integrated on the waveguidefilm. The measured propagation loss of a waveguide in accordance withthis embodiment of the present invention is approximately 0.6 dB/cm at850 nm.

The 45° waveguide mirror coupler (reflective element) is a very criticalcomponent in optical interconnection applications, especially inplanarized lightwave circuits (PLC). The mirror can be incorporated witha vertical optical via to enable 3D optical interconnects and coupleslight to the waveguide. The 45° waveguide mirror is insensitive to thewavelength of light and efficiently couples optical signals fromvertical cavity surface emitting lasers (VCSELs) to polymer waveguidesand then from waveguides to photo-detectors. A very important aspect ofmanufacturing of such coupler is the tolerance interval of the profileparameters, such as the tooth height, the width and the tilt-angle. Thenew fabrication method provided by the present invention uses a verysharp blade, such as a microtome blade, to cut the polymer waveguidesubstrate. A blade sliding down to the waveguide substrate at 45° slopecuts the waveguides at 45°. The process works like a guillotine slidingon a slope. The difference is that the blade of the guillotine falls atthe right angle instead of at 45°.

The material of master waveguide structure is SU-8 ™ (MicroChem)photoresist. The 45° waveguide mirror is fabricated by tilted microtomesetup. The master waveguide structure was kept at 120° C. on a hotplate. In general, elevated temperatures soften the polymer, whichresults in a smoother cutting surface. The blade slides down thesubstrate at 45° slope. The side-off view and surface of the mirror isshown in FIGS. 15A and 15B. All the waveguides shown in FIG. 15A werecut simultaneously by the microtome blade

The coupling efficiency is one of the most critical issues in the fullyembedded optical interconnects because of the concerns about thermalmanagement and crosstalk. Higher coupling efficiency between waveguideand VCSEL or detector enables the lower power operation of VCSEL.Furthermore, when small aperture VCSEL is used to operate at a highspeed, for example, 3 μm aperture for 10 GHz operation, the couplingefficiency is paramount concern because of the large spatial divergenceof VCSEL's light. A large aperture selectively oxidized VCSEL operatesin multiple transverse modes due to the strong index confinement createdby oxide layer with low refractive index. Real spatial distribution ofthe VCSEL is not the same as a Gaussian profile; however, it can beconsidered as a Gaussian profile by ignoring small discrepancies. Thisassumption results in a simple calculation. Another assumption is thatlight within acceptance angle of the waveguide are totally coupled intothe waveguide. There are about 10 supporting modes in the 50 μm squarewaveguide with Δn=0.01. For an exact calculation, all the modes areconsidered, but the number of modes is quite large. It can be treated asgeometrical optics. The coupling efficiency, η can be calculated by theratio of coupled power to total laser power.

$\eta = {\frac{\int_{- r_{c}}^{r_{c}}{{{E\left( {r,z} \right)}}^{2}\mspace{11mu}{\mathbb{d}r}}}{\int_{0}^{\infty}{{{E\left( {r,0} \right)}}^{2}\mspace{11mu}{\mathbb{d}r}}} = {\left( \frac{\omega_{0}}{\omega(z)} \right)^{2}\;{\int_{- r_{c}}^{r_{c}}{{{E\left( {r,z} \right)}}^{2}\mspace{11mu}{\mathbb{d}r}}}}}$where, r_(c) is the maximum radius at the mirror facet which correspondto the acceptance angle of the waveguide.

The coupling efficiencies between VCSEL and square (50 μm×50 μm)waveguide with Δn=0.01 (refractive index difference between core andcladding) were calculated as a function of angular deviation from 45°.The substrate thickness (bottom cladding) and the aperture of the VCSELare 127 μm and 12 μm, respectively. The coupling efficiencies betweenVCSEL and square (50 μm×50 μm) waveguide with Δn=0.01 (refractive indexdifference between core and cladding) are calculated as a function ofangular deviation from 45°. The substrate thickness (bottom cladding)and the aperture of the VCSEL are 127 μm and 12 μm, respectively.

FIG. 16 shows the intensity distributions of laser light at the mirrorsurface, and the coupling efficiencies as a function of angulardeviation from 45° for 127 μm thick substrate, 50×50 μm waveguide, andVCSEL with 12 μm aperture. The facet of 45° mirror is coated with thealuminum to ensure the reflection because TIR (total internalreflection) does not occur due to the top cladding layer. Thereflectance of the aluminum is about 92%. In this scheme all laser lightfalls within the mirror. The coupling efficiency is 92%, which meansnearly 100% of the light is coupled into the waveguide excluding thereflectance due to aluminum. The coupling efficiency maintains constantvalues within 45°±1.5° mirror angle. Therefore, the mirror angle shouldbe kept within 45°±1.5°. The coupling efficiency drops dramatically whenthe mirror angle is out of the tolerance range (±1.5°).

The present invention uses a molding method to fabrication the opticalwaveguide because of its dependable process and suitability for largevolume production. A solid mold is generally used in variousapplications such as embossing, optical disk stamping, and Fresnel lensfabrication. The solid mold is made of nickel alloy by electroplating.The fabrication of the solid mold has higher cost and takes a long time.Alternative mold materials include curable resins, such as silicone, andurethane that can be used to reduce the fabrication cost and time. Thesoft mold has been used in various applications such as rubber stamp,small quantity manufacturing, replication, and micromachining.

Once the master is fabricated, making the mold is simply pouring a moldmaterial over the master and curing. In one example, a siliconeelastomer, especially poly (dimethylsiloxane) (PDMS) is chosen tofabricate the mold. The master for the mold is fabricated on a Si-wafer.Multimode waveguide is required for board level interconnection becauseof the requirement of the lower packaging cost. Alignment of the devices(laser and detector) and the waveguide is easer when the core size ofthe waveguide is large. The size of the multimode waveguide is 50 μm×50μm. The process is the same as standard photo-lithography. The Piranhabath consists of 2 parts of sulfuric acid (H2SO4) and 1 part of hydrogenperoxide (H2O2). A Si-wafer was cleaned in the Piranha bath. After thecleaning, the wafer is baked at 150° C. to remove adsorbed water justbefore spin coating. This baking step improves adhesion and removesbubbles in pre-baking step. After baking, photoresist (SU8-2000,MicroChem™) is poured on the wafer and then spin coated at 400 RPM for 5seconds and then ramped to 1500 RPM for 40 seconds. The wafer is placedon a leveled surface for 5 minutes to improve uniformity. And then, thewafer is moved to leveled hot-plate for a pre-bake. The pre-bake iscarried out at 65° C. for 5 minutes and 90° C. for 40 minutes. Thephotoresist tends to have negative sloped sidewall, which is not goodfor mold applications. The side wall should have positive slope or atleast be vertical for mold application. Exaggerated negative wall isoften called as T-topping. The T-topping results from the lateraldiffusion of the acid near the surface. UV light shorter than 350 nm isabsorbed strongly at the top surface of the photoresist; hence, acid isgenerated by UV, which diffuses laterally on the top surface. TheT-toping can be removed by filtering out short wavelength below 350 nm.Nearly vertical side walls, as shown in FIG. 17A, are made using shortwavelength cut filter (UV-34, Hoya). However, there is still a beakedshaped feature between side walls and top surface. A beaked shaperesults from the diffraction at the interface between the mask and thephotoresist, and it can be eliminated by filling index matching oil(glycerol) into the gap. The ethylene glycol was used to fill the airgap in this experiment instead of using glycerol. As shown in FIG. 17B,the beak was completely removed.

A PDMS mold in accordance with the present invention is shown in FIG.18. The mold material is PDMS (Sylgard 184, Dow Corning). Prepolymer anda curing agent are mixed at 1:10 ratio. Air bubbles trapped in PDMS areremoved in a vacuum chamber. After removing air bubbles, the PDMS ispoured on the master and cured at 90° C. in vacuum chamber for 10 hours.Surface relief structures are transferred from master to the mold.

The fully embedded board level optical interconnection requires a thinflexible optical layer. Current electroplating technology can easilyplate a through-hole or a via having an aspect ratio of 1 in productionline and can plate a hole having an aspect ratio of 3. The size of atypical electrical pad on the device is about 100 μm. These are mainreasons for the thickness limit of substrate film. The thin and flexibleoptical waveguide layer is fabricated by compression molding techniqueusing soft mold. A 127 μm thick optically transparent film (Topas™ 5013)is used as a substrate of the waveguide circuit.

The fabrication steps for the waveguide structure in accordance with thepresent invention are fairly straightforward as illustrated in FIG. 19.The core material (SU-8) is poured on the heated PDMS, which is kept at50° C., in step (a). The heated PDMS mold suppresses bubble generationduring molding process. The excess SU-8 is then scraped out usingsqueegee made of PDMS in step (b). A Topas film is then applied on thetop of the PDMS mold filled with SU-8. The mold and the Topas film areinserted into the press machine and pressure is applied for 30 minuteswhile the plunge plate is held at 90° C. in step (c). The cooling downprocedure is performed wherein the mold pressure decreased gradually dueto the thermal contraction. The core material (SU-8) is transferred tothe substrate film in step (d). Next, the substrate film without the topcladding is exposed to UV to cross-link the SU-8. Once the film isexposed, it becomes chemically and thermally stable. Aluminum is thendeposited on the mirror facets in a vacuum chamber to make the mirror.Finally, top cladding material (Topas) is coated on the film in step(e).

A fabricated optical interconnection layer in accordance with thepresent invention is shown in FIG. 20 has micro-mirror couplers and 12channel waveguides of 50 mm in length. As shown in FIG. 21, the measuredabsorption losses of the Topas material (substrate film) are 0.01 dB/cmand 0.03 dB/cm at 630 nm and 850 nm, respectively. These 10 losses aresignificantly better than previous waveguides (e.g., 6 dB/cm propagationloss at 850 nm for a multi-mode SU-8 waveguide, and 0.22 and 0.48 dB/cmat 1330 and 1550 nm, respectively, using electron beam direct writing).The waveguide propagation loss can be measured using the cut backmethod. The core dimension is 50×50 μm. Fiber pig-tailed 850 nm laser isused to couple the laser to waveguide. The diameter of fiber is 10 μm,which is similar to VCSEL aperture. Coupled out powers according to thelength is shown in FIG. 22. The measured propagation loss is 0.6 dB/cmat 850 nm wavelength. The lights coupled out from 45° mirrors are shownin FIG. 23. A He—Ne laser is lunched at the ends of the waveguides.Lights come out at 45° mirrors which are located at the other ends ofwaveguides.

An example of the device integration process of another embodiment ofthe present invention will now be discussed in reference to FIG. 24.First, a one mil (25.4 μm) thick copper foil is laminated on the top ofthe flexible wave guide layer 2400 by applying heat and pressure 2402.This copper foil is patterned to form the top electrical pads for VCSELand photo-detector. The main reason for the formation of the laminatedcopper foil is the limitation of electroplating. The thickness ofadditional electrical layers easily exceeds 1 mm, and the diameter ofdevice pad is 95 μm. This translates to an aspect ratio of 100;therefore, this hole can not be electroplated. The aspect ration of viacan be reduced by introducing the copper foil just above the waveguidelayer; hence, the micro via can be electroplated. Furthermore, thepatterns on the copper foils can be bigger. This means that largerregistration error can be allowed during laminating process withelectrical layers. The patterns are formed and the back side aligned in2404.

The next step is either laser drilling 2406 or device bonding on thewaveguide layer 2408. There is a possibility of damaging the devicespads during laser micro-via drilling 2412 if the drilling occurs afterthe device bonding 2408. If damage is likely to occur, the devicebonding step 2410 follows drilling 2406. The bonding of devices isperformed using an aligner, which typically has two holders; one formask and the other for substrate. The flexible waveguide film istemporally bonded to a clear glass plate using water and placed on amask holder. The device to be integrated is put on the substrate holder.A small amount of UV curable adhesive is applied on the top of thedevice. When the device and waveguide micro-mirror coupler are aligned,they were exposed with UV to cure the adhesive. The bonding of device towaveguide film can be accomplished by melt bonding without using UVcurable adhesive. When alignment is completed, device is heated justabove the melting temperature of waveguide film for a short period andthe device is bonded to the waveguide film without deforming themicro-vias.

The 30 μm thick, directly electroplated, copper film on the back side ofVCSEL array 2414 is an excellent heat sink that does not sacrifice theuse of easy packaging. Since extremely thin VCSELs are difficult tohandle, 50 μm is the maximum thickness for a reliable operation in afully embedded structure that can be handled by current automated pickand place machines. Smaller sizes can be used as automated pick andplace machines are improved.

FIG. 25 shows a schematic diagram of a flexible waveguide film withoptoelectronic devices that includes a flexible optical waveguide film,12-channel VCSEL array, 12-channel PIN photodiode array and 45°micro-mirror couplers in accordance with the present invention. Aspreviously descrived, the master waveguide structures are formed on asilicon wafer using a standard photo-lithography process. SU8-2050(MicroChem™) is used as the waveguide structures having 12 channelguides with a square shape cross-section (50 μm×50 μm) and a totallength of up to 100 cm. The 45° total internal reflective (TIR)micro-mirrors are adopted to couple light from the VCSELs into thewaveguide array, and then to the PIN photodiodes. To get a soft moldwith 45° micro-mirror couplers, the master waveguide structure is cut onboth ends by a specially designed tool. PDMS (Sylgard 184, Dow Corning)is chosen as a soft mold material. The PDMS is poured on the masterwaveguide structure and cured. Surface relief waveguide patterns with45° micro-mirror couplers are transferred from the master waveguidestructure to the soft mold.

A flexible waveguide film is fabricated by the soft molding process. Thecore material (SU-8) is poured on the heated soft mold and then excessSU-8 is scraped out. The soft mold filled with SU-8 is covered withTopas™ 6015 (cyclo-olefin-copolymer) film, as a bottom cladding layer.The core waveguide structure is transferred from the soft mold to Topas™6015 film using a hot-press machine. A flexible waveguide film withoutthe top cladding layer is exposed to UV light to cross-link the SU-8 andthe surfaces of the 45° micro-mirrors are deposited with aluminum (Al)to ensure the total internal reflection. Finally, the top cladding layeris spin-coated on the film. Measured propagation loss of the waveguideis below 0.5 dB/cm for both TE and TM modes at 850 nm.

Two 12-channel, 850 nm VCSEL arrays (2.5 Gb/s and 10 Gb/s) and a PINphotodiode array are used as I/O sources on a flexible polymericwaveguide film. The initial substrate thickness (200 μm) of the VCSEL isreduced to facilitate thermal management of the VCSEL and the fullyembedded structure. FIG. 26 shows the L-I characteristics of two12-channel VCSEL arrays: 10 Gb/s for both to contacts; and 2.5 Gb/s forthe top and bottom contacts. Apertures of optoelectronic devices areprecisely aligned with I/O windows of the 45° micro-mirror couplers andfixed by a UV curable adhesive. The Electro-optical performancecharacteristics of the 10 Gb/s 12-channel VCSEL array are as follows.

-   -   Threshold Current 0.5˜1.5 (mA)    -   Slope Efficiency 0.35˜0.55 (mW/mA)    -   CentralWavelength 848˜860 (nm)    -   SpectralWidth(RMS) 0.45˜0.6 (nm)    -   Forward Voltage (If=5 mA) 1.4˜2.0 (V)    -   Beam Divergence 27˜32 (deg)    -   Reverse Leakage Current 5˜20 (pA)    -   * Conditions: Tsub=25° C., If=2˜5 mA

FIG. 27 shows an integrated VCSEL and PIN photodiode arrays.Conventional PCB lamination processes are applied to interpose aflexible waveguide film between PCB layers to form the fully embeddedstructure in Sanmina-SCI.

The present invention thus provides an efficient optoelectronic signalcommunications system overcoming process incompatibilities previouslyassociated with implementing optical components in high performanceelectronic systems. While this invention has been described in referenceto illustrative embodiments, this description is not intended to beconstrued in a limiting sense. Various modifications and combinations ofthe illustrative embodiments, as well as other embodiments of theinvention, will be apparent to persons skilled in the art upon referenceto the description. For example, the optical signal communicationssystem may be configured to provide a bus structure, comprising jointtransmit and receive elements fabricated together at a single positionon the waveguide. Further, the principles of the present invention arepracticable in a number of process technologies. It is thereforeintended that the appended claims encompass any such modifications orembodiments.

1. A heat sink for an electrical-to-optical transmitter disposed withina cavity of a printed circuit board, the heat sink comprising a thermalconductive material disposed within the cavity wherein the thermalconductive material is thermally coupled to a bottom surface of theelectrical-to-optical transmitter and at least a portion of the thermalconductive material extends approximately to an outer surface of a layerof the printed circuit board.
 2. The heat sink as recited in claim 1,further comprising a cooling device thermally coupled to the portion ofthe thermal conductive material extending approximately to the outersurface of a layer of the printed circuit board.
 3. The heat sink asrecited in claim 1, wherein the thermal conductive material comprises afilm on the bottom surface of the electrical-to-optical transmitter andan interior wall of the cavity extending approximately to the outersurface of a layer of the printed circuit board.
 4. The heat sink asrecited in claim 3, wherein the film is approximately 30 μm inthickness.
 5. The heat sink as recited in claim 3, wherein the film isapproximately 10 to 50 μm in thickness.
 6. The heat sink as recited inclaim 1, wherein the thermal conductive material comprises copper, athermal conductive paste, or copper and a paste.
 7. The heat sink asrecited in claim 1, wherein the thermal conductive materialsubstantially fills the cavity from the bottom of theelectrical-to-optical transmitter to approximately the outer surface ofa layer of the printed circuit board.
 8. The heat sink as recited inclaim 1, wherein the electrical-to-optical transmitter is between 10 and250 μm in thickness and the heat sink is between 30 and 250 μm inthickness.
 9. The heat sink as recited in claim 1, wherein theelectrical-to-optical transmitter is between 10 and 250 μm in thicknessand the heat sink is between 10 and 490 μm in thickness.
 10. The heatsink as recited in claim 1, wherein the electrical-to-opticaltransmitter comprises a laser, a vertical cavity surface emitting laser,or an edge emitting laser.
 11. The heat sink as recited in claim 1,wherein the electrical-to-optical transmitter disposed within the cavityof the printed circuit board comprises a planarized signalcommunications system.
 12. The heat sink as recited in claim 11, whereinthe planarized signal communications system comprises: a first indexbuffer layer within the printed circuit board; a second index bufferlayer within the printed circuit board; a polymer waveguide disposedbelow the first and above the second index buffer layers; theelectrical-to-optical transmitter disposed within the first index bufferlayer, adjoining the polymer waveguide; a reflective element disposedwithin the polymer waveguide in direct alignment with theelectrical-to-optical transmitter, adapted to reflect optical energyfrom the electrical-to-optical transmitter along the polymer waveguide;an optical-to-electrical receiver disposed within the first index bufferlayer, adjoining the polymer waveguide; a reflective element disposedwithin the polymer waveguide in direct alignment with theoptical-to-electrical receiver, adapted to reflect optical energy fromwithin the polymer waveguide to the optical-to-electrical receiver; andan at least partially metal layer within the printed circuit board,fabricated to provide electrical coupling between theelectrical-to-optical transmitter and a surface of the printed circuitboard, and between the optical-to-electrical receiver and the surface ofthe printed circuit board.
 13. The heat sink as recited in claim 12,wherein the polymer waveguide is fabricated using a compression moldingprocess using UV/thermal cross linkable polymers.
 14. The heat sink asrecited in claim 12, wherein the reflective elements are fabricatedusing a sharp blade.
 15. The heat sink as recited in claim 1, whereinthe electrical-to-optical transmitter disposed within the cavity of theprinted circuit board comprises an optoelectronic signal communicationssystem.
 16. The heat sink as recited in claim 15, wherein theoptoelectronic signal communications system comprises: a substratehaving a first surface; a waveguide structure fabricated within thesubstrate; the electrical-to-optical transmitter fabricated within thesubstrate and in direct adjoinment with the waveguide structure; a firstreflective element fabricated within the waveguide structure in directalignment with the electrical-to-optical transmitter; anoptical-to-electrical receiver fabricated within the substrate and indirect adjoinment with the waveguide structure; a second reflectiveelement fabricated within the waveguide structure in direct alignmentwith the optical-to-electrical receiver; and an at least partially metallayer within the substrate, fabricated to provide electrical couplingbetween the electrical-to-optical transmitter and the first surface, andbetween the optical-to-electrical receiver and the first surface. 17.The heat sink as recited in claim 16, wherein the waveguide structure isfabricated using a compression molding process using UV/thermal crosslinkable polymers.
 18. The heat sink as recited in claim 16, wherein thefirst and second reflective elements are fabricated using a sharp blade.19. A printed circuit board comprising: an electrical-to-opticaltransmitter disposed within a cavity of the printed circuit board; and aheat sink disposed within the cavity wherein the heat sink is thermallycoupled to a bottom surface of the electrical-to-optical transmitter andat least a portion of the heat sink extends approximately to an outersurface of a layer of the printed circuit board.
 20. The printed circuitboard as recited in claim 19, further comprising a cooling devicethermally coupled to the heat sink.
 21. The printed circuit board asrecited in claim 19, wherein the heat sink comprises a thermalconductive material.
 22. The printed circuit board as recited in claim21, wherein the thermal conductive material comprises a film on thebottom surface of the electrical-to-optical transmitter and on aninterior wall of the cavity extending to approximately the outer surfaceof a layer of the printed circuit board.
 23. The printed circuit boardas recited in claim 22, wherein the film is approximately 30 μm inthickness.
 24. The printed circuit board as recited in claim 22, whereinthe film is approximately 10 to 50 μm in thickness.
 25. The printedcircuit board as recited in claim 21, wherein the thermal conductivematerial comprises copper, a thermal conductive paste, or copper and apaste.
 26. The printed circuit board as recited in claim 21, wherein thethermal conductive material substantially fills the cavity from thebottom of the electrical-to-optical transmitter to approximately theouter surface of a layer of the printed circuit board.
 27. The printedcircuit board as recited in claim 19, wherein the electrical-to-opticaltransmitter comprises a laser, a vertical cavity surface emitting laser,or an edge emitting laser.
 28. The printed circuit board as recited inclaim 19, wherein the electrical-to-optical transmitter disposed withinthe cavity of the printed circuit board comprises a planarized signalcommunications system.
 29. The printed circuit board as recited in claim28, wherein the planarized signal communications system comprises: afirst index buffer layer within the printed circuit board; a secondindex buffer layer within the printed circuit board; a polymer waveguidedisposed below the first and above the second index buffer layers; theelectrical-to-optical transmitter disposed within the first index bufferlayer, adjoining the polymer waveguide; a reflective element disposedwithin the polymer waveguide in direct alignment with theelectrical-to-optical transmitter, adapted to reflect optical energyfrom the electrical-to-optical transmitter along the polymer waveguide;an optical-to-electrical receiver disposed within the first index bufferlayer, adjoining the polymer waveguide; a reflective element disposedwithin the polymer waveguide in direct alignment with theoptical-to-electrical receiver, adapted to reflect optical energy fromwithin the polymer waveguide to the optical-to-electrical receiver; andan at least partially metal layer within the printed circuit board,fabricated to provide electrical coupling between theelectrical-to-optical transmitter and a surface of the printed circuitboard, and between the optical-to-electrical receiver and the surface ofthe printed circuit board.
 30. The printed circuit board as recited inclaim 29, wherein the polymer waveguide is fabricated using acompression molding process using UV/thermal cross linkable polymers.31. The printed circuit board as recited in claim 29, wherein thereflective elements are fabricated using a sharp blade.
 32. The printedcircuit board as recited in claim 19, wherein the electrical-to-opticaltransmitter disposed within the cavity of the printed circuit boardcomprises an optoelectronic signal communications system.
 33. Theprinted circuit board as recited in claim 32, wherein the optoelectronicsignal communications system comprises: a substrate having a firstsurface; a waveguide structure fabricated within the substrate; theelectrical-to-optical transmitter fabricated within the substrate and indirect adjoinment with the waveguide structure; a first reflectiveelement fabricated within the waveguide structure in direct alignmentwith the electrical-to-optical transmitter; an optical-to-electricalreceiver fabricated within the substrate and in direct adjoinment withthe waveguide structure; a second reflective element fabricated withinthe waveguide structure in direct alignment with theoptical-to-electrical receiver; and an at least partially metal layerwithin the substrate, fabricated to provide electrical coupling betweenthe electrical-to-optical transmitter and the first surface, and betweenthe optical-to-electrical receiver and the first surface.
 34. Theprinted circuit board as recited in claim 33, wherein the waveguidestructure is fabricated using a compression molding process usingUV/thermal cross linkable polymers.
 35. The printed circuit board asrecited in claim 33, wherein the first and second reflective elementsare fabricated using a sharp blade.